This invention relates to methods for fabricating an 5 high-density-interconnect (HDI) assemblages including components which are damaged by high temperature processing.
High density interconnect assemblages such as those described in U.S. Pat. No. 4,783,695, issued Nov. 8, 1988 in the name of Eichelberger et al., and in numerous other patents, are finding increased usage. In the typical HDI assemblage, a dielectric substrate such as alumina has a planar surface and one or more wells or depressions. Each well or depression extends below the planar surface by the dimension of a component which is to become part of the HDI assemblage. The component is typically an integrated circuit, having its electrical connections or contacts on an upper surface. These contacts or connections are preferably made from titanium-coated copper-containing metals, so that the later formation of through vias by means of lasers exposes titanium, rather than copper, to avoid oxidation of the copper, which oxidation might affect the adhesion of additional layers. Each component is mounted in a well dimensioned to accommodate the component with its contacts in substantially the same plane as the planar surface of the substrate. The components are typically held in place in their wells or depressions by an epoxy adhesive. A layer of dielectric material such as Kapton polyimide film, manufactured by DuPont of Wilmington, Del., is laminated to the devices using ULTEM polyetherimide thermoplastic adhesive, manufactured by General Electric Plastic, Pittsfield, Mass., which is then heat-cured at about 260xc2x0 to 300xc2x0 C. in order to set the adhesive. The polyetherimide adhesive is advantageous in that it bonds effectively to a number of metallurgies, and can be applied in a layer as thin as 12 micrometers (xcexcm) without formation of voids, and is a thermoplastic material, so that later removal of the polyimide film from the components is possible for purposes of repair by heating the structure to the plastic transition temperature of the polyetherimide while putting tension on the polyimide film.
Following the curing of the ULTEM adhesive layer holding the first sheet of dielectric film onto the components, through via apertures are made through the dielectric film and its adhesive layer at the locations of at least some of the electrical connections. The apertures are typically made by the use of a laser. The laser tends to generate soot as the dielectric and adhesive are vaporized. When the connections are made to copper surfaces, the heat of the laser action also tends to create copper oxides on the connections. The soot and oxides tend to prevent good metal-to-metal contact during later stages of processing which include metal deposition.
Following the drilling of through vias through the first layer of the polyimide film and its polyetherimide adhesive, a patterned layer of titanium/copper/titanium electrical conductors is applied to the exposed surface of the polyimide film, into the through vias, and onto the contacts of the components. This completes the formation of a first layer of electrical connections to the components.
One or more additional thin sheets of polyimide dielectric material are layered onto the upper surfaces using silicone polyimide epoxy adhesive (SPIE). The SPIE is a thermoset material such as OXYSIM 600, manufactured by Occidental Chemical Corporation, Grand Island, N.Y., which is then cured at temperatures below 200xc2x0. Once set, the SPIE cannot be softened by heating. Each additional layer of polyimide film has its own pattern of through vias drilled as far as the upper titanium surface of a lower layer of titanium/copper/titanium conductor, followed by its own layer of titanium/copper/titanium deposition. The titanium/copper/titanium layered metallized or deposited conductors are known to provide reliable interconnections.
It has lately become important to integrate into HDI modules certain components including copper-containing electrical connection materials. Such copper-containing electrical contacts are found in at least on-module connection strips, shielding or grounding members, and magnetic components such as tuned ferrite-loaded coils or transformers. These magnetic components tend to be somewhat larger than solid-state chips, but are dimensioned to be accommodated in the HDI modules for which they are intended.
The integration of such modules presents some problems, in that the manufacturers: of the components are accustomed to using copper as the main conductive material, and to making the electrical contacts from copper. Copper is not the best material for electrical contacts in an HDI context, because it oxidizes readily, especially in the presence of high temperatures. Neither titanium nor adhesives reliably adhere to oxidized or dirty copper. Even if they initially appear to adhere, the,adhesion often fails in the presence of heat or moisture. Thus, a copper surface is not acceptable for HDI connection.
Other possible surfaces were evaluated for deposition on the copper of the magnetic components. Electrically deposited and electroless nickel, tin, and palladium were among the surfaces evaluated. It was found that adhesion to nickel was relatively poor for both titanium and adhesive, regardless of how it was deposited. Tin was discounted as a suitable surface, because of the known problem of formation of dendrites. Palladium was also found not to provide good adhesion.
Improved HDI processing methods are desired.
A method for making a high-density interconnection between at least one component and one interconnection sheet includes the step of procuring a component having copper-containing electrical contacts lying in a common plane, and procuring a film of polyimide dielectric material. The method includes the laminating of the film to at least the electrical contacts of the component using a layer of silicone polyimide epoxy adhesive. Following the laminating step, laser-formed vias are defined through the film of polyimide epoxy adhesive, and any polyimide dielectric material which may overlie the contacts, to at least some of the electrical contacts of the component. As a result, soot and copper oxides may undesirably remain on the electrical contacts. At least the vias and that portion of the electrical contacts exposed at the bottoms of the vias are cleaned by at least argon reactive ion etching. Metallization is applied to at least the cleaned vias and contacts, to form a path for the flow of electricity through the film of polyimide;dielectric material to the electrical contacts, and to thereby define the interconnection sheet. In one version of the method, the step of laminating the film to at least the electrical contacts of the component using a layer of silicone polyimide-epoxy adhesive includes the step of curing the silicone polyimide epoxy adhesive. The curing may take place at a temperature not greater than about 190xc2x0 C.
A method for making a high-density interconnection between at least one component and a high-density interconnection sheet according to another aspect of the invention includes the step of procuring a component having copper-containing electrical contacts lying in a common plane, and having a selected dimension in a direction perpendicular to the common plane. In one version, the step of procuring the component includes the step of procuring a magnetic component. The method also includes the step of procuring a dielectric substrate, which may be partially or wholly metallized, and which includes or defines a generally planar surface. The generally planar surface defines a first aperture extending below the planar surface by the selected dimension or otherwise dimensioned to accommodate the component with the common plane substantially coincident with the planar surface. The component is fastened within the aperture, with its electrical contacts lying in a plane substantially coincident with the planar surface. A film of polyimide dielectric material is procured. The film is laminated to at least,the-electrical contacts of the component, and preferably to surrounding portions of the dielectric sheet, using a layer of silicone polyimide epoxy adhesive, and the epoxy adhesive is cured at a temperature not greater than about 190xc2x0 C. Following the curing step, laser-generated vias are formed through the film of polyimide epoxy adhesive and any polyimide dielectric material to at least some of the electrical contacts of the component. If the copper or copper-containing contacts are subject to laser action, soot and copper oxides may undesirably remain on the electrical contacts after the lasering. At least the through vias and that portion of the electrical contacts exposed at the bottoms of the vias are cleaned by at least argon reactive ion etching, to generate cleaned vias and contacts. Following the cleaning, at least the cleaned vias and contacts are metallized to thereby define the interconnection sheet, and to form a path for the flow of electricity through the film of polyimide dielectric material to the electrical contacts.
In one version of the method according to an aspect of the invention, the step of laminating the film includes the step of applying the silicone polyimide epoxy adhesive to a thickness of about ⅕ (10xe2x88x924) meter. In another version, the step of procuring a component includes the step of procuring a component in which the electrical contacts include a copper-containing material coated with titanium, and the cleaning step includes the preliminary step of reactive ion etching with titanium fluoride, to thereby remove the titanium.
In another mode of the method according to an aspect of the invention for fabricating a conductive via between an upper surface of a dielectric sheet and a metallization on a component to which the via is to be connected, a component is procured defining first and second substantially parallel surfaces, defining a thickness therebetween, and including at least one electrical connection material on the first surface thereof, which electrical connection.material includes copper. A substrate defining a planar surface is provided. The substrate defines at least one well extending below the planar surface, which well has a depth substantially equal to the thickness of the component. The component is mounted in the well with the first surface of the component substantially coplanar with the planar surface of the substrate. A sheet of polyimide dielectric material is bonded to at least the first surface of the component. A via is formed using a laser. The via extends through the sheet of dielectric material at a location over the electrical connection material of the component. The formation of the via using a laser undesirably tends to create soot, and also undesirably tends to create oxides of copper on the electrical connection material. At least the via and that portion of the electrical connection material exposed by the laser drilling action are cleaned by steps including
(a) reactive ion etching using a fluorine-containing plasma; and
(b) following the step of reactive ion etching using a fluorine-containing plasma, reactive-ion etching using an argon-containing plasma. Following the step of cleaning the via, metallization is applied to the via and to that portion of the electrical connection material exposed by the via.